Bulk films vs colloidal nanocrystals: the photoactive layers in perovskite solar cells

Perovskite solar cells have attracted significant attention because of their superior photovoltaic performance and promising potential for commercialization. Two distinctive photoactive layers have been developed to realize efficient perovskite solar cells according to the dimension of materials: bulk perovskites and perovskite nanocrystals. Bulk perovskite layers are generally obtained via a traditional nucleation and crystal growth process during film deposition. In contrast, the photoactive layers composed of nanocrystals are prepared through two separate steps: material preparation and film deposition, which decouple the crystallization and film formation process. The differences in the film preparation process determine the different challenges for device assembly. In this review, we systematically compared the film preparation of these two types of photoactive layers and elaborated on the impacts of these processes on solar cell performance. Devices based on bulk perovskites require high-quality polycrystalline films, while devices based on perovskite nanocrystals demand comprehensive surface ligand management for increasing carrier mobility and restraining the surface trap states, as well as giving our perspectives on the future development prospects.